The present invention relates to the field of brazing and, in particular, to a method of making a flux for silver brazing that is non-corrosive and non-hygroscopic in nature, and to a flux cored silver brazing wire and silver brazing paste utilizing the same.
For many years, metal parts have been joined using silver-based brazing compounds. As is well known in the art, it is necessary to prepare the surfaces to be joined prior to applying the brazing compounds in order to provide adhesion of the brazing compound to the surfaces to be joined. This preparation is typically performed by a flux material, which is applied to the joint and activated by the application of heat to the joint. Once activated, the flux thoroughly cleans the surfaces to the joined and removes any oxides that will degrade the strength of the brazed joint.
As they must aggressively clean the surfaces to be joined, fluxes have typically been highly corrosive and hygroscopic in nature. Accordingly, it is necessary in many applications to remove any residual flux or flux residue from the joined parts in order to prevent corrosion of the parts. This removal increases the overall costs of the parts, due to the additional process steps and the cost of waste disposal from the cleaning process. In addition, the waste generated by this cleaning is hazardous to humans and harmful to the environment.
Finally, because of their corrosiveness and affinity for absorbing water, many typical fluxes have not been adapted for use in flux cored wires. As this is the case, the use of these fluxes has necessitated the additional step of applying the flux in a paste form prior to heating and joining the parts. As was the case with the cleaning. step described above, the need to perform this additional step increases the overall cost of the joined parts.
Accordingly, there is a need for a flux for use with silver brazing compositions that effectively prepares the surfaces to be joined, is non-corrosive and non-hygroscopic and, accordingly, does not need to be cleaned from joined surfaces after they are joined, and may be formed into a powder for disposal within a flux cored wire.
The present invention is a silver brazing flux that is non-corrosive and non-hygroscopic in nature, and a method of making this flux. In its most basic form, the flux includes the following compounds in the following percentages by weight:
In addition the above materials, the preferred flux includes boron in the range of 0.01% to 2.0% by weight. The addition of boron in these amounts is preferred as such an addition has been found to enhance the anti-oxidation properties of the flux. However, it is recognized that the flux will work satisfactorily without the addition of boron and, therefore, boron is not included in all embodiments.
In some embodiments, the flux is combined with deionized water, or other suitable binder materials, to form a flux paste. In other embodiments, the flux is mixed sand dried into a powder and dispensed within a silver based filler material using, for example, the process described in U.S. Pat. No. 5,781,846, which is incorporated herein by reference, to form a 5,781,846.
The preferred flux includes the following compounds in the following percentages by weight:
The method of making the flux includes the following steps:
Dispensing the proper percentages of all ingredients by weighing, volume, or other art recognized means. This dispensing step may be performed prior to the mixing steps or contemporaneous with each mixing step.
Adding boric acid on top of potassium bifluoride.
Mixing the boric acid and potassium bifluoride at medium-low speed until a completely smooth wet paste is formed.
Adding potassium tetraborate and mixing, preferably at medium low speed;
Adding potassium fluoroborate and mixing, preferably at medium low speed;
Adding potassium carbonate to the paste and mixing until it is completely dissolved;
Adding boron and mixing, preferably at medium low speed;
Adding a non-reactive liquid, such a deionized water, alcohol, or the like, to each step of the mixture as it begins to stiffen in order to keep a loose, smooth consistency, similar to that of cake frosting, and scraping the sides and bottom of the mixing bowl as needed to keep the mix even.
Mixing the paste, preferably at medium speed.
Stopping the mixer and transferring the mixture into a container, such as a pan. The transferring step preferably involves transferring the mixture such that it fills the container to a depth of between about one inch and about two inches, as filling to this depth allows entrained moisture within the mixture to more easily be exhausted from the mixture during the subsequent heating step and also produces a solid flux that is more easily reduced to powder form.
Drying the mixture by placing the container into an oven preheated to between about 500 and about 700 degrees Fahrenheit and leaving the mixture within the oven for a period of between about two and six hours, such that substantially all moisture is dried from the mixture;
Removing the dried flux and reducing the size of the flux particles to a powder, such as by roll milling, hammer milling, crushing, pulverizing, or the like, and screening to a desired average particle size of between about 20 mesh and about 325 mesh.
The resulting flux powder is non-corrosive and non-hygroscopic, and is readily adapted for suspension within a paste or disposition within a brazing alloy to form a wire brazing composition.
Therefore, it is an aspect of the invention to provide a method of making a brazing flux that is non-corrosive.
It is a further aspect of the invention to provide a method of making a brazing flux that is non-hygroscopic.
It is a further aspect of the invention to provide a method of making a brazing flux that promotes reaction between materials during mixing that dries off moisture while causing dry materials to combine to form a paste material.
It is a further aspect of the invention to provide a method of making a brazing flux that prevents unwanted reactions between materials during mixing.
It is a further aspect of the invention to provide a method of making a brazing flux that does not leave a residue that must be cleaned from surfaces after they are joined.
It is a further aspect of the invention to provide a method of making a brazing flux that effectively prepares surfaces for joining.
It is a further aspect of the invention to provide a method of making a brazing flux that may be formed into a powder.
It is a further aspect of the invention to provide a method of making a brazing flux that may be combined with a silver brazing alloy to form wire brazing composition.
It is a still further aspect of the invention to provide a method of making a brazing flux that may be combined with a binder material to form a flux paste.
These aspects of the invention are not meant to be exclusive and other features, aspects, and advantages of the present invention will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description, appended claims and accompanying drawings.